Verificationism (also known as the Verifiability Criterion of Meaning or the Verification Principle) is the doctrine that a proposition is only cognitively meaningful if it can be definitively and conclusively determined to be either true or false (i.e. verifiable or falsifiable). It has been hotly disputed amongst Verificationists whether this must be possible in practice or merely in principle.

The problem with Verificationism, according to some, is that some statements are “universal” in the sense that they make claims about a possibly infinite set of objects. Since it is not possible to verify that the statement is true for each of an infinite number of objects it seems that verification is impossible.

To counter this, Karl Popper (1902 - 1994) proposed the concept of Falsificationism, whereby if no cases where the “universal” claim is false can be found, then the hypothesis is accepted as provisionally true. A. J. Ayer responded to the charge of unverifiability by claiming that, although almost any statement (except a tautology) is unverifiable in the strong sense, there is a weak sense of verifiability in which a proposition is verifiable if it is possible for experience to render it probable.

Empiricism, all the way back to John Locke in the 17th Century, can be seen as verificationist. The basic tenet of Empiricism is that experience is our only source of knowledge and Verificationism might be seen as simply a consequence of this tenet. Empiricists like David Humerejected philosophic positions about the existence of a God, a soul or even a self, since he was unable to point to (read, verify) the impression from which the idea of the thing is derived. Although the early Empiricists were not directly discussing the meaning of propositions, their general stance was still consistent with Verificationism.

Karl Popper (1902 - 1994) asserted that a hypothesis, proposition or theory is scientific only if it is falsifiable (i.e. it can be shown false by an observation or a physical experiment) rather than verifiable, leading to the concept of Falsificationism. However, he claimed that his demand for falsifiability was not meant as a theory of meaning, but rather as a methodological norm for the sciences.

Some claim that Wittgenstein's Private Language Argument of 1953 is a form of Verificationism, although there is some contention over this. The argument, at its simplest, purports to show that the idea of a language understandable by only a single individual is incoherent.